Originally Posted by Thomathy

Germany appears to be a special case. It consistently flags the 'invalid documentation' message on flights to and from Germany, for me.

A staff member once stated that this is because German border control requires that a passport with a signature field be signed. Therefore a document check is required by the airline to enforce this rule. I don't know how accurate this statement is. For most UK passport holders this is not an issue in any case, as the modern passport has no self-signable section.

I am UK resident and travel on a UK passport. I have only ever seen this message with Germany, where it is consistently shown for me.